In developing a treatment strategy for an infectious disease, a physician must know which antibiotics are most effective in either killing the organism or at least inhibiting or retarding its further growth. The conventional approaches to evaluating the effectiveness of an antibiotic against an organism are typically categorized as: 1) tube dilution assays, and 2) drug diffusion tests.
In the tube dilution assay approach, each antibiotic under consideration is diluted by a series of two-fold dilutions, and the diluted antibiotics are deposited in a set of test tubes. Each tube is then inoculated with the test organism in question and is examined after a 24 hour incubation period to see if the organism has been killed. In this way, the minimal inhibitory concentration is determined. Since data is available about the attainable blood levels for any antibiotic, the minimal inhibitory concentration provides a direct mechanism for determining the potential value and necessary dosage for effective treatment.
The most common drug diffusion test for antibiotic susceptibility is the Kirby-Bauer method. This test involves the use of a Petri plate containing an agar medium whose surface has been swabbed with the test organism. About a dozen disks impregnated with each antibiotic under test are distributed on the surface of the Petri plate. After an incubation period, the diameter of the zone of inhibited growth is measured with calipers. Well-known tables relate the diameter of the inhibition zone to the likely resistance of the organism to the antibiotic.
In many circumstances, the Kirby-Bauer method is preferable to the tube dilution assay approach. Difficulties with tube dilution assays include the fact that certain organisms or media are not amenable to analysis using this method. However, manual measurement of inhibition zones using the Kirby-Bauer method is time consuming and is subject to error.
Systems utilizing image analysis techniques for automating drug diffusion tests have been proposed. G. Hejblum et al in J. Clin. Microbiol., Vol. 31, No. 9, September 1993, pages 2396-2401 describe a system in which the radial profile of each inhibition zone is determined. The shape of the radial profile is analyzed to determine the inhibition zone diameter. Systems using image analysis techniques for automating drug diffusion tests are also described by L. Clontz in "Image Analysis Application to Agar Diffusion Assays", American Clinical Laboratory, August 1992, pages 10 and 11, and by W. Hsia in American Clinical Laboratory, May 1994, pages 28 and 29. The prior art systems are understood to include one or more disadvantages, including the failure of the system to accurately determine inhibition zone diameter for a wide variety of conditions and different organisms.